Abstract: Since the first patent application in the field was officially filled in 19841, stereolithography (SL) has advanced to become a fascinating technological innovation. This technology uses digital data to create complex three-dimensional structures by curing a liquid resin using a UV-laser. The parts fabricated there from have been widely applied for model verification, functional testing and direct manufacturing because of the accuracy, speed and versatility of the SL process. It serves many industries, including service bureaus, consumer products, automotive, medical/dental, motorsports, aerospace and architecture, as well as academic and government/military. The tremendous success of stereolithography came about because of advances in photocurable materials, UV laser technology and 3D computer imaging. This paper will review the different technological achievements that allowed for the growth of stereolithography.

Introduction

There are a number of methods that can be used to manufacture a solid object. One of these methods is subtractive manufacturing. In this method, material is cut away from a starting block. Subtractive manufacturing has many drawbacks. The process generates waste and is time- and cost-intensive. Also, the number of shapes/configurations that can be created is limited. Another technology used to generate objects is formative manufacturing. In formative manufacturing, a resin, polymer or metal is forced into a mold and subsequently cured or allowed to cool. Examples of this include investment casting, injection molding and compressive molding. This technology also has a number of drawbacks, including requiring a labor force that has a broad range of expertise. The process can also be time- and cost-intensive.

Additive manufacturing (AM) technology was developed to alleviate some of the problems of the aforementioned technologies. AM technology builds 3D objects in a layer by layer fashion, significantly reducing manufacturing time and costs without limiting shape configuration. It emerged during the past three decades as a game-changing technology for the rapid prototyping industry. AM technology has recently attracted great attention from public media, including President Obamas 2013 State of the Union address. His administration also invested in the new National Additive Manufacturing Innovation Institute, which will produce 3D printers and create a great number of jobs. AM contains a series of different technological processes, including SL, selective laser sintering (SLS®, a registered trademark of 3D Systems Inc.), three-dimensional printing (3DP), fused deposition modeling (FDM®, a registered trademark of Stratasys Inc.), laminated object manufacturing (LOM) and others.

Stereolithography (SL) is a technology that is used to build solid objects using UV-curable resins. In the building process, parts are built in layers by selectively solidifying photocurable resins. After one layer is built, additional liquid resin is moved over the previously cured layer and then cured. The object is continuously built using this layer by layer approach. These 3D objects can be printed within hours2, 3. This paper will only focus on SL, by giving a brief overview of the technology and reviewing advances in UV-curing during the past three decades.

An overview: Three decades of stereolithography

Wohlers reviewed the early development of stereolithography in detail4. Although the first attempt to build an object in a layer by layer fashion could be dated back to the 1960s, the early concept of modern stereolithography can be dated to 1971, when Swainson presented a patent for a system where two intersecting beams of radiation produced a phase change in a material in order to build a 3D object. Three technologies were combined for the first time to make modern SL usable: computers, lasers and photocurable materials. Unfortunately, the initial technology didnt work at that time and was abandoned because of the lack of funding. In 1981, Kodama published an automated method for fabricating a 3D model in concurrent stages, using a photosensitive polymer. The desired shape of a layer was created using a mask, or X-Y plotter manipulated by an optical fiber. In 1982, Herbert published another new design to create a solid object of any desired cross-section and built a 3D model layer by layer from a photosensitive material. Hull further developed and conceived the idea of modern stereolithography, filed the first patent application of modern SL and coined the term “stereolithography” in 1984. His patent application was granted in 19861. Hull co-founded 3D Systems Inc. and commercialized this new process.

SL greatly benefited in the 1980s from the advancements of computers, UV lasers and UV-curable resins. First, the partnership of Microsoft with IBM allowed for the accessibility of personal computers to the general public in 1980. In the same year, Apple released the Apple III to compete in the market. This led to great advances in computer technology and began the new wave of industry revolution. Secondly, gas-discharged laser technology matured and became commercially available in the 1980s. Although the maintenance cost was quite high, 1.5 Watt of Argon lasers (351.4 nm) could be purchased at an affordable price. Around 1986, the development of a He-Cd laser (325nm) significantly reduced maintenance costs. Thirdly, photopolymer technology improved significantly. DuPont was one contributor in this field, having invented the first photopolymer-based print plate for flexography in 1958. DuPont also commercialized the first dry film photoresist in 1968, which revolutionized PCB (Printed Circuit Board) fabrication methods. This photolithography technology also was a tremendous advancement for the semiconductor industry in the 1980s. Stereolithography synergized with advancements in computer, laser and photopolymerization technology to start and develop the new industry.

There were many players in the early stereolithography machine development market, but most gradually exited or merged because of intellectual property (IP) and funding issues. Today, only a few SL equipment manufacturers dominate the global market. Among these, 3D Systems Inc. provides SL machines globally, while CMET provides machines that are available in the Asian market. Further, Materialise operates its large frame mammoth machine in Belgium, and Shanghai Uniontech sells into the Chinese market.

Three decades ago, SL machines produced only small parts from 1-liter containers, as shown in Figure 1. Today, large frame machines, like the “Mammoth” machine, can consume up to 1.5 metric tons of SL resin and produce a full sized statue. An important reason for this improvement is because of advancements in UV laser technology. In 1996, solid state lasers (354.7nm) were installed in SL systems to replace the low power (20 ~100mW) and high maintenance cost gas discharged lasers. Current 2014 machines incorporate lasers with significantly higher power outputs. For example, a large frame SL machine, the ProJet®8000, is equipped with a 1450 mW solid state laser. Currently, over 7000 mW lasers, with a lifetime of up to 10,000 hours, is commercially available. This can easily meet the future needs of SL machines.

SL applications have expended from the initial prototype design, visualization modeling, planning for manufacturing, and fit and functional testing to tooling and direct manufacturing. SL materials also have advanced from acrylate to hybrid (acrylate and epoxy) systems. Today, novel resins with fillers, reinforcement agents and additives have widened the available SL materials and manufactured material properties to be able to mimic the properties of thermoplastics, ceramics and metals. The SL market has been driven by UV-curable material chemistries and applications, and the sales have increased tremendously with the advancement of SL materials, equipment, software and processing knowledge.

Several factors have driven and will continue to drive stereolithography growth:

Equipment advances that allow for the building of objects at faster speeds, with better accuracy or higher resolution, and in larger sizes.

Lasers which are more powerful, less expensive, have a longer lifetime, are more compact and easier to operate.

Computer capability has increased, while the cost has decreased.

Software is developed to build variable styles and can scan and convert images into building style files faster and easier.

A deeper understanding of photopolymerization, as well as photopolymer physical and mechanical properties.

UV technological innovations have made the SL industry a great success. Among them, UV-curable SL resin innovation is one of the brightest spots over the 30 years. The major material innovation of each decade is reviewed.

The foundation years: 1984-1994

When the modern SL machine was invented, an all-acrylate resin was used. For most of the first decade, resin developers modified and improved these acrylate resins. The major players in the USA included DuPont Somos® and DeSoto (both of which are now owned by DSM under the Somos® brand), Ciba-Geigy SL resins (which became a part of 3D Systems Inc.), and Loctite. The majority of SL materials were invented and developed from knowledge and experience built upon flexography, photolithography materials, photocurable paints and coatings. Acrylate systems provided the required speed and cure properties for early SL. Due to the unique features of SL technology, acrylate SL resins faced several critical technical challenges. The resin viscosities were typically over 2000 mPas at 30&degC, which created limitations. Secondly, dimensional accuracy was problematic, as high volume shrinkage of acrylates caused parts to curl or became distorted after building. Another limitation imposed by acrylates was material properties. SL parts in general were brittle and weak because of resin under-curing, the poor laser power, deficient building styles and limited raw material availability.

In order to support operation and guide formulators, part accuracy assessment methods were developed by engineers and are still widely using for SL material evaluation today. The first method built SL parts with specific geometries, as shown in Figure 2. Part accuracy was assessed by 170 dimensional checks performed on a standardized part geometry using a coordinate measuring machine. Most of the measurements were developed to evaluate two-dimensional accuracy. Then, an H-4 bar method was developed, as shown in Figure 3, to allow for 3D evaluations. When an H-4 bar is built in a vat, lateral shrinkage pulls in the leg and the waist to make the distortion visible, as showed in dotted line of figure 2. This was called a “dent” or “sink” by engineers. Once the constraining supports were removed, a lack of green strength could allow curling of lateral and splaying of legs. After postcure, this distortion developed further.

The volume shrinkage of all acrylate resins varied from 5% to 25%, depending on the backbone structure, functionality and curing conditions. To solve these problems, scientists started to study new chemistries. In 1988, Asahi Denka Kogyo introduced the first epoxy resin for the CMET SL machine in Japan. In 1989, Neckers pioneered one of the earliest research papers on SL material chemistry5, introduced SL technology to the academic field and detailed his vision on application of cationic photopolymerization for SL resins6. In that paper, he also studied visible photoinitiator systems for stereolithography, and pointed out another new opportunity in SL that tailored light to visible wavelength. His group produced the first medical model from a CT scan using stereolithography in the late 1980s7. Cationic photopolymerization has played a dominant role in SL material innovation during the past 20 years, and light wavelength in SL has extended from 325nm, 351nm and 355nm of lasers to 365nm or 405nm LED light or laser. Medical applications also have become one of the most productive SL markets.

In March 1993, Ciba-Geigy, under Max Hunzikers direction in Marly, Switzerland, officially introduced the first epoxide/acrylate hybrid resins in North America8. These hybrid systems represented one of the great milestones in the history of SL material innovations. As shown in Table 1, although the photospeed was slower, epoxy/acrylate systems eliminated the oxygen inhibition completely and reduced the viscosity, which made the process speed faster and cleaning SL parts easier. The green strength and impact strength (or brittleness) also improved with the hybrid system. This new and unique chemistry built interpenetrating polymer networks (IPN) simultaneously, wherein the green parts could be postcured, the dark cure of cationic polymerization continued and the full mechanical properties reached days after the build. This reduced shrinkage stress, eliminating the curl or distortion during building.

The expansion years: 1994-2004

When the first hybrid SL resin was introduced to the US market, it immediately caught the attention of many people. DuPont Somos was acquired by DSM and joined with Japans leading SL material supplier, JSR, in 1999. Building upon their experienced photopolymerization knowledge, the companies led the R&D effort and rapidly expanded SL materials between 1994 and 2004. Many innovations were patented during this decade. The SL materials expansion by mechanical properties of products is demonstrated in Figure 4. X was elongation to break (%), and Y was flexural modulus. These breakthroughs drove the expansion of applications of stereolithography models and the rapid growth of the SL market. Three most important steps of progress were made during this time:

Newly innovated epoxide/hybrid systems combined the materials toughness and stiffness creating SL resins that mimic many properties of popular thermoplastics, such as polyethylene, polypropylene and ABS.

Composite SL materials emerged, dramatically improving build accuracy, expanding the range of strength, stiffness and temperature resistance. DSM Somos® led SL resins innovation at such rapid speed that a series of new products flooded the SL materials market.

As soon as the first hybrid SL resin was commercially available in 1993, the QuickCast™ build style was introduced to make molds from this resin. QuickCast is a method of producing investment casting hollow patterns (Figure 5), minimizing the burn-out material from the core, eliminating the fracture of the ceramic shell and, in the end, generating a superior metallurgical casting. However, first generations of SL parts had poor dimensional stability because of high water absorption. To overcome this, new hybrid SL resins were invented. These had improved water resistance, and the dimensional stability was additionally enhanced (Figure 6). Now many rapid prototyping technologies are able to create direct cast patterns without the initial expense and time required to fabricate a wax pattern tool. However, only stereolithography (SL) provides the dimensional accuracy and the surface finish required for the majority of production investment castings. For this reason, direct casting patterns from SL have become the most common and widely accepted in the investment casting industry.

A new class of SL composite resins also was introduced to the market over this period. These highly filled resins provided for an SL material with a lower coefficient of thermal expansion, lower shrinkage, high modulus and very high HDT (Figure 7). Compared to neat SL resins, these materials enabled novel applications, such as rapid tooling, injection molding (Figure 8) and wind tunnel testing, as well as metal clad composites.

In the mid-1990s, interest in electroplating SL parts9 was enabled by composite SL materials because of the low water absorption and excellent dimensional stability that facilitated structural grade metal plating for enhancement of mechanical performance (Figure 9). This pioneering process is called the metal clad composite (MC2). It is an approach to electroplating a metal clad around an SL inner core, producing metal-like parts that are extremely strong and durable. The metal content by volume is approximately 5% to 20% depending on the type of metal and the performance requirements. MC2 has successfully extended SL applications into high performance prototypes usually created by machining or die casting.

Another spectacular success during this decade was the application of SL materials for invisible dental alignment (Figure 10). Manufacturers adapted and optimized the SL process based on the limited but precise geometry variations during the steps of the teeth alignment process, customized the teeth molds in each step and then used a well-established thermoforming process and materials to create teeth aligners. This innovation enabled the disruptive use of customized retainers in shaping teeth alignments.

The diversification years: 2004-2014

Sales in the SL industry took off after 10 years of foundation building and 10 years of expansion. From 2004 to 2014, innovations in the SL materials fields, driven by collaboration, accelerated SL growth. As a result, sales have soared. SL materials were further diversified by:

The introduction of oxetane chemistry into SL resins, which widened the formulation window.

The introduction of SL resins that matched the performance of certain engineering thermoplastics.

The advancements of clear and composite SL resins.

New specialty SL resins that stretched applications into a micro world and medical fields.

Oxetane, or 1,3-propylene oxide, is a heterocyclic organic compound having the molecular formula C3H6O, Its four-membered ring with three carbon atoms and one oxygen atom evidences a high ring strain energy offering a potentially interesting ingredient for cationic photopolymerizations. Early academic explorations, specifically from the late Wm. Baileys labs at the University of Maryland, showed that commercially available oxetane monomers had relatively slow curing speeds. Soon investigations were reported on the optimization of mixtures of oxetane and epoxide monomers with improved curing speeds.10 SL resin formulators took advantage of these results. The first oxetane-containing formulation was actually commercialized around 2002. Oxetane chemistry widens the formulation window and provides three main benefits:

A lowered formulation viscosity.

Formulations with excellent moisture resistance or high temperature resistances.

Cure and building speeds that were tunable. Oxetanes have thus become essential ingredients in many of SL resins.

The introduction of “X” class SL resins that try to match the performance of engineering thermoplastics as shown in Table 2, also improved the field. The combination of strength, stiffness and property retention pushes “X” class SL resins one step closer to the performance of engineering plastics, Figure 11. This proprietary formulation takes advantage of additives, fillers and hybrid chemistry, and competes in the markets of ABS materials in FDM technology and nylon materials in SLS technology.

A second-generation composite SL resin was commercialized in 2006. This new product took advantage of nanotechnology, improving the settling of first-generation product along with a faster building speed and better aesthetical appearance as shown in Figure 12 than the one in figure 8, this new composite SL resin has been extensively applied in motorsports and wind tunnel model testing.

Low color, high clarity SL materials were introduced in 2001 for niche markets, Following this introduction, a second generation of high clarity SL resins was developed (Figure 13), providing very good dimensional consistency coupled with stable clarity of the cured material and stable viscosity in the vat (Figure 14). These advances expanded the utility of ultra-clear SL resins in consumer items and for automotive prototyping.

It is increasingly essential for medical industries to create customized designs. Stereolithography provides the accuracy needed to fulfill these demands leaving patients with a sense of security and comfort. New clear medical-grade resins produce accurate and clear parts that are ISO 10993-5, ISO 10993-10 and USP Class VI certified. These provide a procedure for tracking and tracing that alert customers of alterations made in the product from raw materials to procedures. The applications include surgical cutting and dental drill guides, in pre-operative anatomical models, as microfluidic biochips in Figure 15, and in master molds for casting implant parts (knee, skull, etc.).

Microstereolithography (MSL), an evolution of stereolithography, is a technology at the interface of the microengineering and stereolithography. Since the resolution of the MSL technique is far better than any of the other technologies, this is of particular interest in microengineering domains where its 3D capability allows the production of components no other microfabrication technique can create. The first developments of the MSL technique began in 199311. In recent years, MSL has become a commercially available manufacturing process. As the market for miniaturized products grows, there is an increased need for high resolution, small prototype parts, to be used as microcomponents for the microrobotic, microfluidic, microsystems and biomedical fields, as well as in jewelry design. New MSL resins allow customers to build layers down to the recorded low of 0.015 of a millimeter, stretching the resolution and accuracy to the highest level as shown in Figure 16.

Conclusion

SL is the most widely used additive manufacturing technology. The market has grown at a fast rate globally and is highly competitive today. This growth demands that SL resin suppliers innovate aggressively and rapidly. Future SL materials still have many areas in which to improve technically. This includes temperature resistance and toughness in combination with stiffness, stable mechanical properties, color, and faster imaging and recoating processing. Many opportunities exist for the future development of SL machines as well, such as SL systems based on even longer wavelength lasers or LED light (365nm or 405nm), or digital light projecting technology (DLP®, a registered trademark of Texas Instruments Inc.). It is anticipated that the efficiency of future equipment will cause an advance in general purpose SL resins, and specialty SL materials will advance significantly as applications expand.

Acknowledgments

The framework of information in this paper was provided by Jim Reitz, business director at DSM Functional Materials (2000~2010). Consultation on the early SL history provided by Ed Murphy, scientist at DSM Functional Materials (1974~2011); Dr. Daniel Mickish, R&D manager of Somos Solid Imaging Materials Group in DuPont or DSM (1991~2004); and Dr. Douglas Neckers, currently CEO of Spectra Group Limited, professor (emeritus) and founder/director of the Center for Photochemical Sciences at Bowling Green State University (1985~2009).

References

C. Hull, Apparatus for production of three-dimensional objects by stereolithography, US4575330, 1986.

Dr. Kangtai Ren is a senior scientist working on optical fiber coatings at DSM Functional Materials. He joined DSM Somos in 2005, having worked on stereolithography technology for over six years. Ren received his Ph.D. in organic chemistry from Nankai University in China. He has been published in more than 30 publications and has 10 issued patents or published patent applications. Contact Kangtai Ren at kangtai.ren@dsm.com.